The invention relates generally to a method and system for recovering oil from an oil-bearing formation. More particularly, the invention relates to a method and system for recovering oil from an oil-bearing formation by injecting a water stream having controlled salinity content.
Low salinity water is typically injected into an oil bearing reservoir in order to enhance the oil recovery from such reservoirs. The enhanced oil recovery is typically effected due to an increase in wettability of the rock matrix, thereby, releasing extra oil from the reservoir. However, one of the challenges associated with this method is that currently available desalination techniques yield water of a salinity that may be lower than the optimal salinity for enhanced oil recovery. In fact, the desalinated water produced by such techniques may actually be damaging to the reservoir and inhibit oil recovery, for example by causing swelling of clays. Thus, there is an optimal salinity for the injection water that provides the benefit of enhanced oil recovery while avoiding formation damage, and, the optimum value may vary from formation to formation.
Typical methods of optimizing the salinity of the water stream include blending the low salinity water stream with a water stream of high salinity (for example, sea water). However, it may not be desirable to mix a desalinated water of low multivalent cation content with a high salinity water (such as seawater), because of the high sulfate anion content and/or the high multivalent cation content of sea water. The high sulfate anion content of such blended water streams may result in reservoir souring and/or the precipitation of unacceptable levels of insoluble mineral salts (scale formation). Further, the presence of multivalent ions in the water above a certain concentration level may also have negative effects, such as, precipitation of surfactants.
Some other methods of optimizing the salinity content include splitting the seawater stream into two water feed streams and utilizing two filtration processes: reverse osmosis (RO) and nano-filtration (NF), followed by blending of the two treated streams down-stream of the RO unit. Such systems and processes, however, may have the drawbacks of one or more of higher energy requirements, a larger system footprint, higher complexity of controls, and higher complexity of operation. Forward osmosis (FO) is another method sometimes utilized to generate water stream with controlled salinity content. However, the FO process uses a draw solute to artificially increase salinity on the permeate side, and thus requires a chemical system to remove the draw solute before discharging the permeate.
Thus, there is a need for improved methods and systems for producing water streams having controlled salinity content, which is suitable for injection into an oil-bearing formation. Further, there is a need for improved methods and systems for recovering oil from an oil-bearing formation by injecting a water stream having the controlled salinity content.